Offshore drilling rigs or platforms may be attached to the ocean bottom or, in substantially deeper water, may be floating types held in a desired position through the use of a plurality of anchors. Since each corner pillar typically may have two anchors and anchor lines, a minimum of eight such anchor lines need to be deployed when the platform is put into position and recovered to permit the platform to be removed. Each such line, particularly if over 500-600 feet in length, usually consists of a length of wire rope attached to a length of chain at the end of which is an anchor. Wire rope has better characteristics than chain with respect to strength per unit of weight but does not, alone, provide a good suspension means for an anchor of the standard type because it tends to be quite stiff, imposing upward forces on the anchor when unloaded and preventing the flukes from digging into the bottom as desired. Thus a length of at least a few hundred feet of chain below the wire rope and next to the anchor tends to provide the best arrangement for proper anchor operation, the chain being attached to a connector between the wire rope and the chain.
In U.S. Pat. No. 3,842,776 filed in the name of Donald J. Wudtke (common assignee), an anchoring system for our offshore platform is described in which a special connector is combined with a special fairlead sheave designed to smoothly guide chain, connector and wire rope in both directions without kinking or otherwise damaging the wire rope. When the anchor lead is being retrieved, the wire rope is wound on a drum by means of a winch until the connector is in a desired position adjacent a work platform or station, at which time a "devil's claw" or similar chain stopping mechanism is attached to one of the chain links and the winch is reversed to put slack in the chain above the devil's claw. A detachable link is then opened to permit the wire rope and connector to be removed and to permit a length of chain from a separate chain locker and windlass to be attached to the anchor chain. The windlass is then operated to draw the chain into the chain locker.
The use of the devil's claw arrangement has certain disadvantages as to cost, maintenance and, primarily, safety. Such devil's claw members are normally attached to the platform with wire rope or chain which is somewhat lighter in weight than the anchor cable/chain combination because their only function is to support the weight of the anchor chain and anchor for a short period. With twelve or more such devices fastened to the outside of the platform and exposed to the elements, including salt water spray, there is a danger that one or more may deteriorate to the point where they may break, permitting the entire anchor chain and anchor to drop to the bottom. At the same time, parts of the devil's claw and/or some chain links may swing or whip in unexpected directions, thus constituting a hazard to personnel at the work station involved with the anchor deployment and retrieval operation.